The invention concerns a test element analytical system for the analytical examination of a sample and in particular a body fluid of humans or animals. The system comprises at least two components, i.e., a test element which has a measuring zone into which the sample to be examined is moved in order to carry out an analysis in order to measure a measurable variable that is characteristic for the analysis, and an evaluation device with a test element holder in order to position the test element in a measuring position in order to carry out the measurement and a measuring device for measuring the characteristic measurable variable.
Test element analytical systems are commonly used especially in medical diagnostics for analysing body fluids such as blood or urine. The sample to be examined is firstly applied to a test element. Here the process steps that are required to detect the analyte which are usually chemical, biochemical, biological or immunological detection reactions or physical interactions take place which result in a characteristic and measurable change of the test element especially in the area of the measuring zone. In order to determine this characteristic change the test element is inserted into an evaluation device which determines the characteristic change of the test element and provides it in the form of a measured value for display or further processing.
Test elements are often designed as test strips which are essentially composed of an elongate support layer, usually made of a plastic material, and a measuring zone with a detection layer containing detection reagents and, if necessary, other auxiliary layers such as filtration layers. The test elements of the present invention additionally contain contact areas which can be used to make an electrical contact between the test element and the evaluation device. In the case of electrochemical assay methods conductor paths and electrodes are mounted on the test element. Even test elements which do not use methods of electrochemical analysis can have electrically conducting contact areas for example in order to transfer calibration data or batch information that are stored on the test element to the evaluation instrument.
The accompanying evaluation devices have test element holders with special contact elements which make an electrically conducting contact between the test element and the measuring and evaluation electronics of the evaluation instrument. These contact elements are usually in the form of electrical plug connections with metallic spring elements which are often provided with a noble metal surface usually of gold or platinum. The test strips are inserted into the test element holder for the measurement during which the contact areas of the contact elements are moved across the electrodes of the test elements. In an end position the contact area of the contact elements of the evaluation instrument are then in contact with the contact area of the test element. An electrically conducting connection is made between the test element and evaluation instrument by a pressing force that is in particular defined by the shape and spring force of the contact element. This should in particular ensure that the transition resistance between the contact area of the contact element of the evaluation instrument and the contact area of the test element is as low and constant as possible to enable an exact and reproducible signal transfer. A constant and reproducible transition resistance is especially important in order to still obtain exact measurement results even after a test element has previously been plugged in many times and thus to obtain a high and reproducible measurement accuracy especially with regard to the fact that such test element analytical systems are often used for many years or many tens of thousands of plugging operations are carried out. This is of major importance especially in the clinical field where such test systems often have to handle a high throughput.
A major advantage of pluggable contact devices is the ability to easily join and separate the electrical connection so that the test element and evaluation device can be stored and used independently of one another. Since the contact areas should, on the one hand, ensure that the transfer of electrical current is as optimal as possible which requires a certain contact pressure, but, on the other hand, joining the contact connection and in particular repeated joining and separating the contact connection puts a great strain on the connection, the contact areas are often provided with a layer of noble metal for example by plating or galvanizing with gold, silver, platinum or palladium. The often high mechanical strain on the contact areas especially due to abrasion, deposition or scratching of the contact areas is thus also a problem because a certain contact pressure has to be ensured for a reliable electrical contact and a certain insertion path of the test element is necessary for mechanical reasons and in particular to ensure guidance when plugging in and mechanical stability in the plugged state. It is very important that the contact areas are as resistant as possible to external influences in order to make a very secure contact between the contact areas of an electrical contact connection and with regard to having the lowest possible contact resistances. In this connection the external influences can be of a chemical, physical or mechanical type. Thus, especially during the plugging process, the two contact areas rub against one another resulting in a very high mechanical strain. Corrosion effects and especially crevice corrosion also have an adverse effect on the contact security and contact resistance. Another problem of such test element analytical instruments is that the support material of the test elements that are used often consists of an elastic and relatively soft plastic foil on which the contact areas and electrodes are mounted so that this structure on a relatively soft base material can have disadvantages for an exact contacting.
A major disadvantage of noble metal-noble metal pairs for contact areas of such plug-in connections is that, even irrespective of their geometry and/or the pressing force, the metal surfaces are very often damaged when the contact areas are joined and thus electrical contact problems occur. Such contact problems often manifest themselves in that the transition resistances between the plug and contact element become very high or in an extreme case there may be no longer any electrical contact between the components of the contact connection. When observed under the microscope the picture of damage that often results, especially in the case of flat contacts such as conductor paths or electrodes, is characterized by a major change in the thickness of the metal layer of these contact areas after the insertion. Thus the metal layer of the electrodes is strongly deformed in some areas by the second contact area that moves across it, in particular in the form of grooves, ridges and scratches. This pattern of damage occurs especially when the electrodes are mounted on relatively soft base materials. These deformations may become so large that the metal layer is completely stripped away in some areas by the second contact area moving across it. In this case electrical contact between the test element and evaluation instrument is no longer possible. Such deformations of metal layers which serve as contact areas manifest themselves as non-defined and considerably increased transition resistances or in the complete lack of an electrical contact. Such contact elements are therefore unsuitable for use in analytical systems which are intended to ensure a reproducible determination of analyte over a long period of use.
Hence in order to overcome these disadvantages the following solutions have been given in the prior art:
In order to ensure a very secure contact of plug-in connections especially under high mechanical and/or chemical stress, DE 102 22 271 A1 describes a method for increasing the mechanical and/or chemical resistance of an electrical contact connection between two contact parts by coating at least one of the contact parts with the aid of a thermal spraying process in the area of the contact areas. The aim of this application is to minimize the wear of the contact area by this coating. It mentions plug-in connections of electronic components such as conductor boards and printed circuit boards, or sliding contacts for example in motors as fields of applications for such contact connections. Such contact connections are especially characterized in that after the involved contact areas have been contacted once, the contact connection is subjected to a continuously high mechanical strain for example by vibrations or continual grinding together of the contact areas resulting in a large amount of wear of the involved contact areas in this area. The object of this application is in particular to minimize the wear on the contact areas themselves rather than to ensure a reliable electrical contact of the contact areas even after multiple joining and separation of the contact connection. Hard-wearing metal alloys such as bronze are mentioned as coating materials which are applied to one or both contact areas in order to thus reduce the wear on these contact areas themselves. The coating itself is carried out using thermal spraying processes. Such processes which use high temperatures are unsuitable for test elements whose test supports are very often composed of thin plastic foils since such plastic foils do not have the necessary heat resistance. The layer thickness of the coating layer has to be relatively large at 10 μm to 200 μm in order to enable a durable connection even under high strain and to enable the still unavoidable wear. Such increased wear phenomena occur in particular when both contact areas are provided with such a hard-wearing coating.
The European Patent Application EP 0 082 070 also describes a process for protecting electrical contact connections especially in switches and relays. The aim of this application is to protect metals and especially metal contacts from wear by coating. Like DE 102 22 271 A1 the coating should make the contact areas more resistant to wear. For this purpose a layer of titanium nitride is applied to the existing metal contacts which is characterized by the following features: an adhesion of more than 180 kg/cm2, high chemical resistance, high abrasion resistance and a specific resistance of ca. 500 μΩ*cm. Also in this case the coating is used to minimize the wear of the contact areas themselves rather than to ensure a reliable electrical contact of the contact areas even after multiple joining and separation of the contact connection.
U.S. Pat. No. 6,029,344 describes spring contact elements especially for electrically contacting electronic components which are coated with a hard material. The aim is to modify the mechanical properties of the contact connection by the coating of the hard material. This is especially intended to improve the elastic properties of the contact element. In this case the coat is not used primarily to reduce the wear of the contact areas or to make a more secure contact, but rather to modify the elastic properties of the spring contacts. For this purpose the spring contacts made of relatively soft base materials such as gold are coated with a material which has a higher yield strength than the base material at least in the areas that are shaped in such a manner that they allow a spring action of the contact element. Examples of such materials that are mentioned are in particular metals such as nickel, copper, cobalt, iron, gold, silver, elements of the platinum group and other noble metals, semi-noble metals, tungsten, molybdenum, tin, lead, bismuth and indium and alloys thereof. These materials are referred to as hard materials in the sense of U.S. Pat. No. 6,029,344 and are defined as materials which have a yield strength of greater than 80,000 psi. Hard materials are defined completely differently in the sense of the present application. Such hard materials according to U.S. Pat. No. 6,029,344 are not suitable for ensuring the requirements with regard to a very high abrasion resistance and high contact reliability even for multiple insertions but rather serve to improve the elastic properties of the spring contact. The layer thicknesses of the hard material coating have to be between ca. 6 and 125 μm and have to be at least one fifth to five-fold the layer thickness of the base material of the spring contacts according to U.S. Pat. No. 6,029,344 in order to improve the mechanical and in particular the elastic properties of the contact element.
The documents described above describe processes for coating surfaces of electrical contact elements which either serve to reduce the abrasion and the wear of the contact areas themselves or to improve the elastic properties of the contact element. A fundamental problem which cannot be satisfactorily solved by the aforementioned processes and devices is to ensure a reliable and defined electrical connection between the contact areas of a contact element over a long time period especially under high mechanical strain and even after numerous contacting operations.